1. Field of the Invention
The present invention relates to river and sea water intake screens with circular filter panels to prevent ingress of debris and particles carried by the water.
2. Description of the Prior Art
A water intake screen is generally disposed a few centimeters from the downstream side of a grid of bars protecting it from large debris, either on its own or in conjunction with one or more other screens of the same type forming part of the same screening station.
In the prior art, the filter element, which has a mesh size of only a few millimeters, is generally mobile so that it can be periodically cleared of debris and particles with dimensions greater than the mesh size, which progressively obstruct the whole of the mesh and therefore clog the filter-element.
This kind of mobile filter element may comprise a panel adapted to be raised in its own plane alternately with another panel of the same type disposed parallel to and a distance from it.
This kind of filter element may equally consist of an endless loop of elongate cross section constituting a filter chain known as a direct passage or double flow filter chain.
This kind of filter element may also consist of an endless loop of circular or polygonal cross section constituting a filter drum.
Be this as it may, this kind of mobile water intake filter element, whether it be a screen with a filter panel, a filter chain or a filter drum, is moved cyclically from an immersed position, in which it is progressively charged with debris and diverse particles, to a non-immersed position, in which counterflow pressurized water jets clear it of particles and debris accumulated on its surface in this way, so that on subsequent re-immersion it is again able to exercise its filtering function.
The particles and debris entrained by the washing water are usually collected in bulk in a channel provided for this purpose via which they are evacuated to a drain.
If there are any fish on the panels, they are removed from the water and suffer impacts that usually kill them.
Similarly, leaks may occur between the panels themselves or between the panels and fixed lateral uprights of the filter frame.
Moreover, fish trapped or entrained by the mobile filter element of this kind of water intake screen are inevitably removed from the water during the non-immersed phase of the filter element, often for a long period of time, and are subject to the usually violent action of the washing water jets applied to the filter element and to the resulting violent splashing in the channel for recovering the particles and debris temporarily clogging the filter element in question.
Direct passage chain filters, i.e. chain filters in which only the upstream face of the filter apron receives water to be filtered, also have various disadvantages, as follows:
Firstly, it is necessary to provide a sealing plate known as a footplate at the bottom of the sluice, at the base of the filter apron, to provide a seal between the apron and the bottom of the sluice, and this plate must be tangential to said apron.
Now, because of inevitable manufacturing and assembly tolerances and wear of components, and in particular wear of the apron support chains, there is always a minimum clearance between this kind of footplate and the filter apron; in practice, this clearance is never less than around 6 mm and often even larger.
As a result, with a view to consistency, the filter aprons of these direct passage chain filters cannot employ a mesh size of less than 6 mm, which limits the effectiveness of cleaning.
In double flow chain filters, the filter apron has generatrices parallel to the direction of flow of the stream of water to be filtered and the two faces of the filter apron exercise identical roles in parallel; said stream flows either from the outside to the inside of the filter apron across both faces thereof or in the opposite direction.
The supporting frame of this kind of double flow chain filter is usually disposed against facing masonry walls of the sluice.
These double flow chain filters have the advantage that, since no seal is required at the base of their apron, they allow the use of small mesh sizes, for example of less than 0.5 mm, with no inconsistency with any other lack of sealing.
They also have the advantage of allowing simultaneous use of both faces of their apron and therefore of doubling the effective filtering area, i.e. the capacity of the apparatus, for an equal volume.
However, double flow chain filters offer no improvement in terms of the survival of fish as compared to direct passage chain filters.
Various devices have already been proposed for equipping a water intake screening station with means for saving living creatures carried by the water taken in, and in particular fish. However, in installations of this type known at present, these devices are either special filters, which in themselves merely save fish without having any general screen function, or conventional screen filters, which means that any fish swept up remain out of the water for some time.
Thus a general object of the present invention is to enable the required screening of water intakes without the drawbacks of earlier structures and in a manner that is safe for fish.
Moreover, in the case of existing installations equipped with direct passage chain filters, the problem increasingly arises of improving cleaning efficiency by reducing the dimensions of the debris or organisms that can be blocked.
At present, however, substituting plant providing at least the same functionality without the same drawbacks and without modifying the civil engineering works or sluice hydraulics is no easier, the material conditions for implementation thereof being different.
Another general object of the present invention is to facilitate such substitution, if required.